Problems associated with the formation and discharge of electrostatic charge during the manufacture and utilization of photographic film and paper have been recognized for many years by the photographic industry. The accumulation of charge on film or paper surfaces leads to the attraction of dust, which can produce physical defects. The discharge of accumulated charge during or after the application of the sensitized emulsion layer(s) can produce irregular fog patterns or "static marks" in the emulsion. The severity of static problems has been exacerbated greatly by increases in the sensitivity of new emulsions, increases in coating machine speeds, and increases in post-coating drying efficiency. The charge generated during the coating process results primarily from the tendency of webs of high dielectric polymeric film base to charge during winding and unwinding operations (unwinding static), during transport through the coating machines (transport static), and during post-coating operations such as slitting and spooling. Static charge can also be generated during the use of the finished photographic film product. In an automatic camera, the winding of roll film out of and back into the film cassette, especially in a low relative humidity environment, can result in static charging. Similarly, high-speed automated film processing can result in static charge generation. Sheet films are especially subject to static charging during removal irom light-tight packaging (e.g., x-ray films).
It is generally known that electrostatic charge can be dissipated effectively by incorporating one or more electrically-conductive "antistatic" layers into the film structure. Antistatic layers can be applied to one or to both sides of the film base as subbing layers either beneath or on the side opposite to the light-sensitive silver halide emulsion layers. An antistatic layer can alternatively be applied as an outer coated layer either over the emulsion layers or on the side of the film base opposite to the emulsion layers or both. For some applications, the antistatic agent can be incorporated into the emulsion layers. Alternatively, the antistatic agent can be directly incorporated into the film base itself. The requirements for antistatic layers in silver halide photographic films are especially demanding because of the stringent optical requirements, the layers must be both highly transparent and essentially colorless.
Other types of imaging elements such as photographic papers and thermal imaging elements also frequently require the use of an antistatic layer. For these types of imaging elements, the antistatic layer is typically employed as a backing layer. For photographic paper, an additional critical criterion is the ability of the antistatic backing layer to receive printing (e.g., bar codes or other indicia containing useful information) typically administered by dot matrix or inkjet printers and to retain these prints or markings as the paper undergoes processing (viz, backmark retention).
Electrically-conductive layers are also commonly used in imaging elements for purposes other than providing static protection. Thus, for example, in electrostatographic imaging it is well known to utilize imaging elements comprising a support, an electrically-conductive layer that serves as an electrode, and a photoconductive layer that serves as the image-forming layer. Electrically-conductive agents utilized as antistatic agents in photographic silver halide imaging elements are often also useful in the electrode layer of electrostatographic imaging elements.
Colloidal metal oxide sols which exhibit ionic conductivity when included in antistatic layers are often used in imaging elements. Typically, alkali metal salts or anionic surfactants are used to stabilize these sols. A thin antistatic layer consisting of a gelled network of colloidal metal oxide particles (e.g., silica, antimony pentoxide, alumina, titania, stannic oxide, zirconia) with an optional polymeric binder to improve adhesion to both the support and overlying emulsion layers has been disclosed in EP 250,154. An optional ambifunctional silane or titanate coupling agent can be added to the gelled network to improve adhesion to overlying emulsion layers (e.g., EP 301,827; U.S. Pat. No. 5,204,219) along with an optional alkali metal orthosilicate to minimize loss of conductivity by the gelled network when it is overcoated with gelatin-containing layers (U.S. Pat. No. 5,236,818). Also, it has been pointed out that coatings containing colloidal metal oxides (e.g., antimony pentoxide, alumina, tin oxide, indium oxide) and colloidal silica with an organopolysiloxane binder afford enhanced abrasion resistance as well as provide antistatic function (U.S. Pat. Nos. 4,442,168 and 4,571,365).
Conductive layers employing electronic conductors such as conjugated polymers, conductive carbon fibers, or semiconductive inorganic particles have also been described. Trevoy (U.S. Pat. No. 3,245,833) has taught the preparation of conductive coatings containing semiconductive silver or copper iodide dispersed as particles in an insulating film-forming binder. Such coatings, although they provide excellent conductivities, impart some color to the imaging element and are, therefore, undesirable in many photographic applications.
Conductive fine particles of crystalline metal oxides dispersed with a polymeric binder have been used to form substantially transparent conductive layers for various imaging applications. Many different metal oxides, such as ZnO, TiO.sub.2, ZrO.sub.2, SnO.sub.2, ZnSb.sub.2 O.sub.6, Al.sub.2 O.sub.3, BaO, etc, have been described for use in electrically conductive layers as mentioned in U.S. Pat. Nos. 4,275,103, 4,393,441, 4,416,963, 4,418,141, 4,431,764, 4,495,276, 4,571,361, 4,999,276, 5,122,445, and 5,368,995, for example. In order to obtain high electrical conductivity, a relatively large amount of metal oxide must be included in the conductive layer. This results in decreased optical transparency for thick conductive coatings. The high refractive index (&gt;2.0) of the preferred metal oxides necessitates that the metal oxide be dispersed in the form of ultrafine (&lt;0.1 .mu.m) particles, prepared by various mechanical milling processes, in order to minimize light scattering (haze) by the antistatic layer. The cost for these metal oxide materials and the cost involved in the milling process required to obtain ultrafine particle size make the preparation of such conductive layers rather expensive.
As indicated above, the prior art on electrically-conductive layers in imaging elements is extensive and a very wide variety of different materials have been proposed for use as the electrically-conductive agent. There is still, however, a critical need in the art for improved electrically-conductive layers which are useful in a wide variety of imaging elements, which can be manufactured at reasonable cost, which are durable and abrasion-resistant, which are effective at low coverage, which are adaptable to use with transparent imaging elements, which are colorless, and which do not exhibit adverse sensitometric or photographic effects.